Hydrocarbons have many valuable uses, including fuels for transportation and feedstocks for industrial processes. Hydrocarbons can be obtained via the processes of reservoir characterization, management, and production (including enhanced oil recovery). These processes depend on engineers' understanding of, or assumptions about, the phase behavior of the reservoir fluids. If the phase behavior is not properly understood, reservoir production strategies will not work as intended. For example, excess fluids may suppress desired gas flows, or inadequate gas expansion may be unable to drive desired fluid flows. Conversely, proper understanding of the phase behaviors enables reservoir fluids to be manipulated for optimum production volumes, rates, and/or efficiency.
The current standard for characterizing the phase behavior of reservoir fluids is pressure/volume/temperature (PVT) measurement. PVT measurement is conducted in bulk pressure cells and thus only bulk fluid PVT data are available. However, this conventional PVT technique may not be applicable for unconventional resources, such as nano-confined liquid- or gas-rich shale systems.
Shale reservoirs have demonstrated great potential to supply the world's energy needs in the future. It remains challenging to evaluate the shale reservoir rock, partially due to the abundant ultra-narrow pores on nanometer scale and complex mineralogy. See Musharfi, N., Almarzooq, A., Eid, M., Mahmoud, J., Mahmoud, J., Buller, D., et al. (2012), Combining Wireline Geochemical, NMR, And Dielectric Data For Formation Evaluation And Characterization of Shale Reservoirs, SPWLA 53rd Annual Logging Symposium, 16-20 June, Cartagena, Colombia, SPWLA-2012-074. In addition to common minerals, a significant amount of organic matter is found in intraparticle pores. See Loucks, R. G., Reed, R. M., Ruppel, S. C., & Hammes, U. (2012), Spectrum of pore types and networks in mudrocks and a descriptive classification for matrix-related mudrock pores, AAPG Bulletin, 96, 1071-1098. Nanoscopic pores develop within organic matter during the thermal maturation process. Such organic nanopores may induce large capillary pressures and influence the phase behavior of the hydrocarbon fluids due to the nanoscopic confinement effect. See Gelb, L. D., Gubbins, K. E., Radhakrishnan, R., & Sliwinska-Bartkowiak, M. (1999), Phase separation in confined systems, Reports on Progress in Physics, 62, 1573. Nanoconfined fluids have depression of the bubble point pressure, resulting in lesser volume of gas liberation at any given pressure below the observed saturation pressure. See Honarpour, M. M., Nagarajan, N. R., Orangi, A., Arasteh, F., & Yao, Z. (2012), Characterization of Critical Fluid, Rock, and Rock-Fluid Properties-Impact on Reservoir Performance of Liquid-rich Shales, SPE Annual Technical Conference and Exhibition, (p. SPE 158042), San Antonio, Tex., USA. For gas condensate systems on the other hand, there are conflicting observations with regard to depression or elevation of dew point pressure point in small confined spaces. Compare Civan, F., Michel, G., & Sigal, R. (2013), Impact of Fluid Behavior Modification under Elevated Pressure and Temperature Conditions on Shale-Gas/Condensate Reservoir Engineering and Production Analysis, SPE Unconventional Resources Conference-Canada, (p. SPE 167186), Calgary, Alberta, Canada; and Nojabaei, B., Johns, R. T., & Chu, L. (2013), Effect of Capillary Pressure on Phase Behavior in Tight Rocks and Shales, SPE Reservoir Evaluation & Engineering, SPE-159258-PA.
Conventional PVT measurement is in the bulk phase, and is thus unable to probe the true phase behavior and fluid composition inside the shale nanopores. Previous groups have attempted numerical simulations to assess the phase behavior of hydrocarbon fluids in shale nanopores. However, the equation of state for these nanoconfined systems remains an issue of controversy. See Zhang, Y., Civan, F., Devegowda, D., Jamili, A., & Sigal, R. F. (2013), Critical Evaluation of Equations of State for Multicomponent Hydrocarbon Fluids in Organic Rich Shale Reservoirs, Unconventional Resources Technology Conference, (p. SPE 168812/URTeC 1581765), Denver, Colo., USA. No consensus has been reached on how the phase behavior responds to the nanoscopic confinement. See Pang, J., Zuo, J., Zhang, D., & Du, L. (2013), Effect of Porous Media on Saturation Pressure of Shale Gas and Shale Oil, International Petroleum Technology Conference, (p. IPTC 16419). Beijing, China. There has been no lab characterization method available to directly assess the phase behavior of reservoir fluids in shale reservoir rocks with microscopic details. This deficiency can significantly impact estimation of hydrocarbon storage and recoverability.